Multilayer polyester films are known in the prior art.
U.S. Pat. No. 5,484,632 describes an unoriented multilayer film comprising a first layer made of amorphous, crystallizable polyethylene terephthalate with a melting point above 238° C., and with enthalpy of fusion about 9 cal/g, and a second layer made of amorphous, heat-sealable polyester which consists essentially of monomer units of terephthalic acid, about 85-93 mol % of ethylene glycol, and about 15-7 mol % of cyclohexanedimethanol and which has enthalpy of fusion greater than 0.5 cal/g, where the thickness of the second layer is less than 50% of the total thickness of the film.
U.S. Pat. No. 5,968,666 discloses a film with at least seven layers, comprising first and third layers and also, arranged between the first and third layer, a second layer which comprises a terephthalic acid polyester and, in at least one direction, has been oriented with a degree of stretching that is higher than the degree of stretching achieved under identical stretching conditions with a monolithic film made of terephthalic acid polyester. It is preferable that the first and/or third layer comprise(s) a polyethylene naphthalate polyester.
Certain types of polyester have for some years been used to increasing extent as material for packaging films and shrink films, because the thermal and optical properties are more advantageous than those of PVC. A material that has in particular proven successful for shrink films is a semicrystalline or amorphous polyester of the EMBRACE® or EMBRACE® LV types from Eastman Chemical Company, where a CHDM glycol component is used as comonomer alongside terephthalic acid and ethylene glycol. The shrinkage performance of shrink films made of the EMBRACE® group of polyesters is approximately the same as that of shrink films made of PVC. In the case of shrink films based on the EMBRACE® group of polyesters it is possible via targeted adjustment of process parameters and/or admixture of modifiers to adapt the shrinkage curve within certain limits to the user's requirements, where the shrinkage curve describes shrinkage as a function of temperature: by way of example, a shrink film based on EMBRACE® LV can be adjusted so that its shrinkage curve is markedly flatter than that for conventional PETG-type copolyester, for example PETG 6763 from Eastman Chemical Company, and also exhibits considerably lower shrinkage stress.
In the context of large-volume industrial-scale trials using shrink labels based on the CHDM-glycol-modified polyesters, the inventors of the present application have observed a defect/reject rate amounting to a number of percentage points. Many of the shrink films based on said polyesters, and shrink labels produced therefrom cause problems during processing, or exhibit shortcomings in the printed image. A number of causes were indentified following detailed defect analysis:                blocking of the shrink film during unwinding after print pretreatment by means of corona discharge. These blocking problems lead to whitish surface cracks/markings, for which the term “blocking” or “ghosting” is used below. Blocking defects or ghosting defects remain visible after printing and moreover in unfavorable situations cause tearing of webs during unwinding;        uncontrolled angular distortion of the film during shrinkage, for which the term “twist” or “bowing” is used below; and        on occasion inadequate adhesion of the adhesive bond; the adhesive bond is produced when solvent is used for the adhesive bonding of the shrink film to give a tube.        
It is an object of the present invention to provide a shrink film which                consists essentially of polyester;        has shrink performance similar to that of shrink films made of PVC; and        in further processing has a defect rate and reject rate of less than 1%.        